Speaker protective circuit

ABSTRACT

A speaker protective circuit is provided designed to be interposed between the speaker transducer and driving signal source. The protective circuit includes a silicon controlled rectifier shunted across the transducer adapted to fire in response to either repetitious, intermittent power surges or continuous high level surges and a circuit breaker in series connection with the driving signal source adapted to open circuit in response to long duration power surges or a series of closely spaced short duration surges.

United States Patent Pauly [54] SPEAKER PROTECTIVE CIRCUIT [72] inventor: Walter W. Pauly, Teaneck, NJ.

[73] Assignee: Stanton Magnetics, Inc., Plainview,

' L.I.,N.Y.

[22] Filed: April 16, 1971 [21] Appl.No.: 134,703

[52] US. Cl ..307/202, 307/237, 307/252 W, 307/252 H [51] Int. Cl. ..H02h 7/20, H03k 5/08 [58] Field of Search...307/202, 252 S, 252 N, 252 H, 307/237, 252 W [56] References Cited UNlTED STATES PATENTS 3,215,896 11/1965 Shattuck et al. ..307/202 X 1 Aug. 22, 1972 3,317,792 5/ 1967 Sutherland ..307/202 X FOREIGN PATENTS OR APPLICATIONS 1,477,358 3/1967 France ..307/202 Primary Examiner-John Zazworsky [57] ABSTRACT A speaker protective circuit is provided designed to be interposed between the speaker transducer and driving signal source. The protective circuit includes a silicon controlled rectifier shunted across the transducer adapted to fire in response to either repetitious, intermittent power surges or continuous high level surges and a circuit breaker in series connection with the driving signal source adapted to open circuit in response to long duration power surges or a series of closely spaced short duration surges.

1 Claim, 1 Drawing Figure SPEAKER PROTECTIVE CIRCUIT BACKGROUND OF THE DISCLOSURE transducers have an input voltage limit after which areing and dielectric breakdown occurs, they must be provided with some effective means of limiting their input to protect them from going into nonlinear operation. In addition, overloading of any acoustical speaker can result in permanent damage to the speaker or possibly, to the ears of the listener.

Heretofore, various schemes have been suggested to prevent the overloading of speakers. The fuse or circuit breaker, both of which are relatively slow acting devices, have been used to open circuit the speaker from its power source after a threshold level driving signal has been exceeded. Both these devices operate by heating a sensing element and thus, are more or less time dependent, having an inherent time lag determined by the heating time required. Thus, while the fuse type limiting device is effective for relatively longterm power control, it is ineffective for instantaneous control (i.e., where there is a short duration voltage surge).

In certain telecommunication applications, the function of limiting the input voltage is performed by a device which, instead of opening a circuit, shunts the available current and short circuits the excitation potential. These devices are available in various forms of biased diodes and they are frequently used to limit transients in noisy power lines. The use of biased diodes, however, produces very high levels of distortion, particularly if the input signal has high level transients which are relatively short in duration. For a communication system not primarily concerned with distortion-free sound reproduction, the use of biased diodes has proved to be economical and efficient. However, the use of this protective method in a high quality audio reproduction device has heretofore been un- S UMMARY OF THE INVENTION The above and other beneficial objects and advantages are attained in accordance with the present invention by providing aaspeaker protective circuit adapted to be interposed between the output of a speaker driving device and the input to a speaker transducer connected to the driving device. The protective circuit comprises fuse means connected in series with the driving device and a silicon controlled rectifier shunted across the speaker input. A first resistor series connected to a second resistor is shunted across the driving device output and the gate of the silicon controlled rectifier is connected to the junction of the first and second resistors.

The output signal of the speaker driving device is sampled by the voltage divider defined by the first and second resistors. If the potential at the junction of the resistors exceeds a predetermined threshold level, the silicon controlled rectifier will be fired, thereby shorting the speaker input substantially instantaneously. The silicon controlled rectifier remains conductive as long as the sampled signal exceeds the threshold level, thereby keeping the speaker shorted. If the speaker is shorted out for a sufficiently long time, the fuse (circuit breaker) will open thereby permanently disconnecting the speaker from the driving device.

A capacitance device may be shunted across one leg of the voltage divider to store part of the sampled signal for a short time duration so that the fuse will open by virtue of either a relatively long duration power surge or a closely spaced series of instantaneous peaks.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing is a schematic representation of the speaker protective circuit of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the accompanying drawing, the protective circuit 10 of the present invention is shown interposed between the output of an audio amplifier l2 and a speaker 14. While the present protective circuit may be used with either dynamic or electrostatic speakers, it is particularly well suited for use with electrostatic speakers since such speakers are especially susceptible to damage from high voltage surges and such surges could do permanent damage to the speaker diaphragm.

Amplifier 12 includes an output terminal 16 and return terminal 18 which is commonly maintained at ground potential. Speaker 14 is provided with a pair of terminals 20 and 22 which ordinarily are connected to the amplifier output terminals 16 and 18.

As stated before, the speaker protective circuit 10 of the present invention is interposed between amplifier l2 and speaker 14. The protective circuit includes a fuse member 24 connected in series with the amplifier output terminal 16. Fuse 24 may comprise any device which open circuits when the current passing through it exceeds a predetermined level. Although a conventional fuse comprising a strip of wire that melts when the level is exceeded may be used, an electromagnettype circuit breaker is preferred since the circuit breaker may readily be reset and the circuit completed without necessitating the replacement of a component. The output of amplifier 12 is fed to speaker 14 through fuse 24 and speaker input terminal 20. The resistance of fuse 24 is represented in the schematic by resistor 26.

A voltage divider, formed of two series connected resistors 28 and 30, extends between the junction of fuse 24 and the-speaker input 20 and the return lead 32. A capacitor 34, shunted across resistor 30, and a silicon controlled rectifier 36 complete the present protective circuit. The anode 38 of the SCR is connected to the junction of resistor 26 and speaker terminal 20 and the cathode 40 is connected to the return lead 32. The SCR gate terminal 42 is connected to the junction of resistor 30 and capacitor 34. A diode 44 is provided in series with resistor 28 insuring the polarity of the voltage applied to capacitor 34.

In operation, the driving signal from amplifier 12 to speaker 14 is sampled via resistors 28 and 30. Part of the sampled signal is rectified and stored in capacitor 34. The value of the stored signal is determined by the ratio of resistors 28 and 30. When capacitor 34 charges to the point where it gates the silicon controlled rectifier 36, the SCR becomes conductive thereby shorting out the speaker. The current through fuse 24 will then rise sharply so that if the silicon controlled rectifier remains fired for a sufficiently long time, fuse 24 will open thereby disconnecting speaker 14 from amplifier 12. Similarly, a series of peaks spaced sufficiently close to one another to maintain the charge on capacitor 34 at a level to trigger SCR 36 will have the same effect, Thus, after a sufficient number of such peaks, fuse member 24 will also open circuit.

The response time for fuse member 24 and the signal level at which SCR 36 is triggered is determined by the value of the components which, in turn, are determined by the chosen threshold values. In practice, it is suggested that the present circuit be designed so that the SCR fires in response to an output signal of approximately 110 dB sound pressure and the fuse member have a response time of approximately 500 mil-v liseconds. As stated, the response of theSCR is substantially instantaneous so that the interruption of signals above the threshold level is sufficiently rapid that the shorted signal is virtually unheard by the listener.

Thus, in accordance with the above, an improved (audio speaker) protection circuit is provided. It should be realized that although a specific embodiment of the invention has been described and illustrated, various changes therein may become evident to one skilled in the art which do not depart from the scope of the invention as set forth in the appended claims.

Having thus described the invention, what is claimed 15:

l. A speaker protective circuit interposed between a speaker driving device and a speaker transducer element, said protective circuit being adapted to prevent audio signals from said driving device above 'a predetermined cumulative level from passing to said transducer element, said circuit comprising: fuse means connected in series with said driving device; a silicon controlled rectifier shunted across said speaker transducer element; a first resistor and a diode series connected to a second resistor connected in parallel with said driving device and said speaker transducer element, said diode having an anode connected to said first resistor and a cathode connected to said second resistor, the value of said resistors being such that the potential at the junction of said second resistor and said diode cathode is sufficient to gate said silicon controlled rectifier when the output level from said driving device reaches said predetermined level; connecting means extending between the gate terminal of said silicon controlled rectifier and said junction; and, a capacitor shunted across said second resistor whereby to maintain the potential at said junction for a predetermined time period. 

1. A speaker protective circuit interposed between a speaker driving device and a speaker transducer element, said protective circuit being adapted to prevent audio signals from said driving device above a predetermined cumulative level from passing to said transducer element, said circuit comprising: fuse means connected in series with said driving device; a silicon controlled rectifier shunted across said speaker transducer element; a first resistor and a diode series connected to a second resistor connected in parallel with said driving device and said speaker transducer element, said diode having an anode connected to said first resistor and a cathode connected to said second resistor, the value of said resistors being such that the potential at the junction of said second resistor and said diode cathode is sufficient to gate said silicon controlled rectifier when the output level from said driving device reaches said predetermined level; connecting means extending between the gate terminal of said silicon controlled rectifier and said junction; and, a capacitor shunted across said second resistor whereby to maintain the potential at said junction for a predetermined time period. 